1. Field of the Invention
This invention relates generally to an apparatus for compressing and transmitting signals, and more particularly to a signal compressing apparatus and a signal transmitting apparatus for selectively compressing audio signals to provide high-quality sound or compressing voice signals so as to shorten a time delay due to compression processing.
2. Description of the Prior Art
A variety of techniques for high efficiency encoding of audio signals have been previously proposed. These techniques may be exemplified by sub-band coding (SBC) which is a non-blocking frequency band dividing technique for dividing, without blocking, audio signals in the time domain into plural frequency bands and encoding them. The so-called transform coding is a blocking frequency band dividing technique for converting signals in the time domain into signals in the frequency domain by spectral transform, so as to divide the signals into plural frequency bands and then encode each band.
Also, a technique for high-efficiency encoding comprised of a combination of the sub-band coding and the transform coding has been considered. In this technique, after the band division is carried out by the sub-band coding, the signal of each band is spectral-transformed into a signal in the frequency domain, and then each spectral-transformed band is encoded. A so-called QMF filter may be used as a filter for the band division.
The spectral transform is for blocking input audio signals by a predetermined unit time, that is, a frame, and converting the time domain signals into the frequency domain by carrying out the fast Fourier transform (FFT), the cosine transform (DCT), and the modified DCT transform (MDCT). The MDCT is described in J. P. Princen, A. B. Bradley, Univ. of Surrey, Royal Melbourne Inst. of Tech., "Subband/Transform Coding Using Filter Band Designs Based on Time Domain Aliasing Cancellation," ICASSP 1987.
By quantizing the signals which have been thus divided for each band by the filter and the spectral transform, it is possible to control the bands in which a quantization noise is generated and to carry out encoding of higher efficiency in terms of auditory sense by utilizing characteristics such as masking effects. If each band is normalized at the maximum of the absolute value of the signal component of the band prior to the quantization, it is possible to carry out encoding of still higher efficiency.
As for the frequency division width for quantizing each frequency component produced by the frequency band division, band division taking account the characteristics of the human auditory sense is carried out. More specifically, there are several cases in which the audio signal is divided into a number of bands, for example, 25 bands, with the bandwidth of a so-called critical band being wider toward the higher frequency band. Encoding data of each band in this case is carried out by predetermined bit allocation or by adaptive bit allocation. For example, in encoding by the bit allocation method the coefficient data produced by the MDCT processing, the MDCT coefficient data of each band are encoded with the adaptive number of the allocated bits.
Such high-efficiency compressing encoding is appropriate for transmitting audio signals of high quality without widening the transmission band. For example, when a system is to be constructed by which audio signals are listened to separately through headphones or earphones at a number of seats in a vehicle such as an aircraft, and wherein audio signals can be freely selected for each seat from among the audio signals of multiple channels, it is desirable to employ the high efficiency compression coding as described above in order to realize multiple-channel transmission without causing deterioration of the quality of the audio signals.
In the above-described high efficiency compression coding technique in which a high compression rate and high sound quality are maintained, it is often the case that a relatively long time is required for compression and expansion, thus generating a time difference between the compressed/expanded signals and those directly transmitted without being compressed/expanded. For this reason, in case the high-efficiency compression coding is applied to an audio system of headphone-listening type provided for each seat of a vehicle such as an aircraft, a time difference is caused between the audio signals through the headphones and the sounds in an announcement from the cockpit through PA speakers provided on the walls and ceilings. This causes difficulty in listening to and understanding the announcement.
When a message from the cockpit is announced in the aircraft, the channel selection status of audio signals at each passenger seat is mandatorily switched over to a predetermined PA channel, while the announcing voice in output through the PA speakers, so as to assure delivery of the message to the passengers listening over the headphones at the time of the announcement. Therefore, if a delay due to the compression/expansion time is generated in the announcement voice through the headphones, while the announcement voice through the PA speakers is directly reproduced as a normal analog transmission without having a temporal delay, the time difference between these voices causes an apparent echo, thus resulting in difficulty in hearing causing a loss of part of the words at the time when the PA channel is mandatorily selected.
In addition, if one were simply to compress/expand the announcement using the high-quality sound technique, when the compressed voices are transmitted to speakers in the cabin, the person announcing the message hears his or her own voice through the speakers. Therefore, the large temporal delay may cause confusion.
In view of the above-described status of the art, it is an object of the present invention to provide a signal compressing apparatus and a signal transmitting apparatus by which audio signals such as music are maintained at a high quality, while voice signals such as a message to be announced are compressed/expanded in a short time, so as to have a small temporal delay.